Mulder - Spender's Brother?

Though neither of them know it, Mulder and Spender may be brothers. CSM is Spender's father, and he may be Mulder's as well. Tena Mulder, Mulder's mother, had an affair with CSM, and he still carries a torch for her. When CSM is shot in "Redux II" and thinks he is dying, his last act is to clutch a childhood photo of Mulder and his sister Samantha. It's not clear whether he is the father of Fox, Samantha, or both, but he does seem to have some interest in protecting Mulder. And in "Musings of a Cigarette-Smoking Man," when everyone else is going home to their families for Christmas, CSM pauses wistfully outside the door of Mulder's office.

Like many siblings, Spender and Mulder don't get along. Spender blames Mulder for the disappearance of his mother, Cassandra. And Mulder blames Spender for the shooting of his former lover, Agent Fowley. Spender actually had nothing to do with Diana Fowley's shooting, but Mulder refuses to believe that. He is convinced that Spender has been suborned by CSM, because he saw them together. It was perfectly innocent - CSM approached Spender, who had no idea who CSM was, or why Mulder should be so upset to see them talking. Mulder, perhaps because he was previously burned when his partner, Alex Krycek, turned out to be a Consortium agent, nevertheless believes the worst of Spender, at one point even physically attacking him.

Their relationship only deteriorates as Spender falls under the influence of his father, CSM - fulfilling Mulder's dark suspicions. Spender tries to block Mulder out of the X-files, and Mulder goes around him. Spender shreds case files, and Mulder goes through his trash and tapes them together. When Cassandra Spender is returned, she refuses to speak to anyone but Mulder. Reluctantly swallowing his pride, Spender asks Mulder to help him. But Mulder thinks it's a trap, and refuses. Then Spender catches Mulder and Scully breaking into his office, and gets them suspended from the FBI. Meanwhile, CSM is telling Spender that he "pales to Fox Mulder."

Eventually, however, Spender comes to realize that Mulder is right - about the aliens, anyway. He asks for a meeting with Mulder, Scully, Skinner, and Kersh, where he announces that Mulder is right and should be put back on the X-files. Spender then resigns from the FBI.

Whatever Mulder used to think about Spender, by the end, he's realized that the kid's okay. He is plainly affected by Spender's declaration of support. The way Mulder winces when Spender announces he's resigning - clearly, if he'd had the opportunity, Mulder would have tried to talk him out of quitting. Unfortunately, they never got that chance.